The study of ions by cyclotron resonance techniques continues to expand almost sixty years after its introduction. The many types of cyclotron experiments all rely on the calculation of ion mass from its inverse relationship to the measured cyclotron frequency in a given magnetic field. Earlier ion cyclotron resonance (ICR) experiments scanned through a single frequency at a time and were, thus, relatively time consuming. The advent of more sensitive detection electronics and fast Fourier transformation by computers led to Fourier transform ion cyclotron resonance (FT/ICR) experiments, which obtain the entire frequency spectrum at once. Chemical interest in ICR goes well beyond mass spectrometry. The ion selectivity afforded by double resonance techniques, where certain ions are purged from the cell by ICR excitation while other ions are monitored, is useful in observing reaction rates. All ICR experiments rely on the ion trapping due to cyclotron motion. The magnetic field confines ions transversely in cyclotron orbits, and the addition of electrostatic plates leads to harmonic axial motion. This combination of electric and magnetic fields keeps ions trapped as long as several seconds in typical chemical applications. To study single trapped ions, however, physicists employ precise field configurations such as the Penning trap: a single electron has been trapped for as long as ten months.
While ICR and related techniques are highly sensitive, the only observable they measure is mass. Conventional ICR thus cannot distinguish two structurally different species of equal mass. This is in marked contrast to nuclear magnetic resonance (NMR) and electron spin resonance (ESR), which yield a wealth of information from low-energy spectral features, but have poor sensitivity. This is a basic problem of experimental chemical physics and extends to other forms of spectroscopy; discrimination and sensitivity seem incompatible.
Because of the long collisionless periods obtainable, high resolution optical spectroscopy of trapped ions is an active area of research. It relies on spontaneous emission and photon counting, neither of which are practical at the low frequencies of primary interest here. More closely related to the present invention is work in which electrical signals associated with the ion motion have been used to detect spectroscopic events at low frequencies where direct spectroscopy is impractical. One way this has been achieved is to electrically detect the loss of ions that results from spin-dependent ion loss from a spin-polarized ensemble of ions. Dehmelt and Majors detected the ESR of He ions in this way in an rf quadrupole trap using collisions with a polarized Cs atom beam. In a conceptually related scheme, Richardson, Jefferts and Dehmelt obtained ESR spectra of H.sub.2.sup.+ by taking advantage of the spin-dependence of photodissociation in this molecule.
The experiment most similar to the present invention is the detection of the ESR of a single electron by Van Dyck, Wineland, Ekstrom and Dehmelt. In this work a magnetic bottle field was superimposed on a Penning trap and the axial motion of a trapped electron at 4.2K was monitored as spin flips were induced with microwaves. A spin flip from one spin state to the other showed up as a shift in the axial frequency. The magnitude of this frequency shift was about 1 Hz, near the resolution limit of the technique. Since the observed shift is predicted to be inversely proportional to particle mass, it would be difficult or impossible to do this experiment on an ion.
The present invention also uses frequency shifts in ion motion induced by a magnetic bottle to detect the internal spectroscopy of trapped charged particles. It differs from the electron experiments in several important ways. First, the ion motion detected is the cyclotron motion, rather than the axial motion. This is preferable because it has the highest frequency (making it easier to detect with adequate signal-to-noise ratio) and because it provides the best mass resolution. Secondly, the shift detected is not the small one directly associated with the flip of the spin magnetic moment, but rather one associated with a change in the axial motion of the ion. This axial motion is itself made spin-dependent by a second and distinct use of the magnetic bottle field: a sequence of one or more spin flips synchronized with the axial motion is shown to induce a cumulative increase or decrease in the energy of the axial motion depending on the initial spin state.
The following prior art references provide background information relevant to the present invention.
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